During the past several decades, investigative efforts for developing planar antennas have been expended because this class of antennas has inherent merits such as low cost of manufacture, ease of manufacture, compactness, and lightweight. These antennas in being planar and having low profiles are thus suitable for many industrial or military applications where the size and shape of antennas are crucial. This holds true especially for antenna array applications. However, the planar antenna as a resonant structure seriously suffers from the problem of having a narrow impedance bandwidth. Therefore, many proposals have been developed in an attempt to alleviate the narrow impedance bandwidth problem.
A microstrip patch antenna forms one type of planar antennas, where a conducting metal plate known as a patch is separated from a ground plane by a dielectric to form parallel planes. Patch antennas are attractive because of inherent characteristics such as low cost of manufacture, conformability, and ease of manufacture. However, a major limitation in the implementation of this type of antennas in various applications lies in the inherently narrow impedance bandwidth achieved, which is typically only in the order of a few percent when such antenna patches operate as radiators. For practical applications of antennas, a wide impedance bandwidth is generally required. On account of this fact, several techniques have been proposed to increase the impedance bandwidth of patch antennas. For a patch antenna having a single patch radiator, techniques involving the addition of a parasitic patch, the use of electrically thick substrate, and the cutting of a thin slot in the patch radiator have been proposed.
A parasitic patch increases the patch antenna volume substantially and is complicated to implement in an array design involving patch antennas. As to the method of using an electrically thick substrate in a patch antenna, the problem arising from using a long probe as a feed to the patch radiator has to be taken into account. The large inductance stemming from the long probe limits the achieved impedance bandwidth to less than 10%. In addition, undesired surface waves resulting from antenna operation decrease the radiation efficiency of the patch radiator significantly. By cutting a small circular slot to introduce capacitance or adding other capacitive components around the long probe or between the long probe and the patch radiator, an impedance bandwidth of 10% is obtainable. With the introduction of a U- or L-shaped slot, a substantial increase in impedance bandwidth of more than 20% is achievable. The impedance bandwidth of this kind of slotted patch radiator, however, is sensitive to the dimensions and locations of the slots.
As a variation to the patch antenna with electrically thick substrate, a patch antenna suspended above a ground plane has also been proposed. In this proposal, however, there is a tradeoff between the size and complexity of the antenna, and the impedance bandwidth of the patch antenna. By significantly enlarging the size and complicating the design of such a patch antenna, the impedance bandwidth may reach 65%. The spacing between the patch and the ground plate ranges from 0.14 .lambda. to 0.27 .lambda. (.lambda. is the operating wavelength) and the area of the patch also ranges from 0.39.times.0.36 .lambda..sup.2 to 0.70.times.0.76 .lambda..sup.2. Additionally, the dimensions of a finite-size copper plate used to approximate an infinitely ground plane ranges from 0.75.times.0.56 .lambda..sup.2 to 1.46.times.1.10 .lambda..sup.2. To alleviate this tradeoff, there is a further proposal to use an L-shaped probe as a feed for reducing the spacing between the patch and the ground plane, which ranges from 0.08 .lambda. to 0.15 .lambda.. However, the achievable impedance bandwidth is approximately 30%.
In yet further proposal, a stub is applied to a patch antenna where the stub is used specifically for tuning the resonant frequency for antenna operation and for matching the patch antenna. In this proposal, the stub is applied to the patch antenna for dual or triple band operations with narrow impedance bandwidths.
There are a number of proposals which have been disclosed in patents in which the impedance bandwidths of proposed patch antennas have been extended. By using matching devices in one such proposal, the impedance bandwidth of a suspended patch antenna reaches 20%. Based upon the use of a parallel Resistance Capacitance Inductance (RCL) model in another such proposal, the design of a broadband patch antenna having series lumped capacitive devices is achievable. By tilting a radiating patch at an angle, the impedance bandwidth of another further proposed patch antenna is approximately 10%.
In proposals disclosed in patents for broadening impedance bandwidths of patch antennas, various techniques are used. A proposal described in U.S. Pat. No. 4,605,933, issued to The United States of America as represented by the Secretary of the Navy on Aug. 12, 1986, involves a large spacing (0.1.about.0.25 .lambda.) between a radiator and a ground plane essentially for achieving an extended impedance bandwidth. However, matching devices must be introduced to the proposed antenna structure. Additionally, a non-planar ground plane is utilised, which is not suitable for array applications.
Similarly, having a complicated mechanical structure also poses a problem for a patch antenna as described in U.S. Pat. No. 4,835,539 issued to Ball Corporation on May 30, 1989. The use of lumped capacitive devices greatly increases manufacture cost therein. Furthermore, this type of proposed antenna structure is not suitable for array applications.
The approximately 10% impedance bandwidth achievable by a patch antenna described in U.S. Pat. No. 5,734,350, issued to Xertex Technologies, Inc. on Mar. 31, 1998, does not generally satisfy the requirement for wide impedance bandwidth in practice industrial applications. Additionally, a tilted plate implemented in the proposed antenna structure is not easily fabricated and mounted for array applications.
U.S. Pat. No. 5,874,919 issued to Harris Corporation on Feb. 23, 1999 describes a stub proposed for use as a tuning element for an active stacked patch antenna. This impedance bandwidth broadening technique disclosed mainly depends on both proximate feeding and stacked elements rather than the attached stub.
Among proposals for increasing the impedance bandwidth of a planar antenna, there are proposals for achieving broad impedance bandwidths using plate antennas. In these proposals, plate antenna broadbanding methods are employed for purposes of realizing the broad impedance bandwidth of a planar structure. However, these proposed planar antennas have complicated mechanical structures, which therefore adversely offset to a great extent the advantage gained in having broad impedance bandwidths.
One such planar antenna is described in U.S. patent application Ser. No. 08/669,047, where a radiator plate 102 with a special shape notch 104 and to which is attached a slant parasitic sheet 106 on the plate bottom as shown in FIG. 1. The proposed mechanical structure is complicated in respect of the design and adjustment process for such an antenna. Furthermore, the radiator 102 is completely suspended and a coaxial probe 108 (for example from a commercial service mount adapter (SMA) connector 110) is used for feeding the radiator 102 through the slant parasitic sheet 106. This kind of antenna is difficult to fabricate and assemble.
Another such planar antenna is proposed and disclosed in an article"Broadband Microstrip Antenna" by Luk et al. (Electron., Lett., Vol. 34, No. 15, PP. 1442-1443, 1998). The design of such a proposed antenna, as shown in FIG. 2, suffers from drawbacks such as complexity of mechanical structure, particularly in the implementation of an L-probe 202 which is spaced apart from a radiator 204, resulting in manufacture, adjustment and installation difficulties. Consequently, the complicated proposed mechanical structure undesirably increases the manufacture cost.
In the other proposals for achieving broad impedance bandwidths for planar antennas, one such proposal for a planar antenna being shown in FIG. 3, stubs are applied to patch antennas having narrow impedance bandwidth. The stubs provide specific load matching functions. As proposed in an article "Tuning Stubs for Microstrip-Patch Antennas" by M. Plessis and J. H. Cloete (IEEE Antennas and Propagation Magazine, Vol. 36, No. 6, PP. 52-55, 1994), and shown in FIG. 3, a radiator 302 has stubs 304 extending from the peripheries, and a coaxial probe 306 for feeding the radiator 302. The coaxial probe 306 directly contacts the radiator 302.
There is therefore a need for a low profile, cost effective and easily manufactured planar antenna having extended impedance bandwidth.